Method of manufacturing semi-conductive devices having electrodes containing aluminum



May 4, 1965 N. B. SPEYER 3,181,226

METHOD OF MANUFACTURING SEMI-CONDUCTIVE DEVICES HAVING ELECTRODES C AINING ALUMINUM Filed June 1959 NICO BRAM SPEYEFL United States Patent M 3 181 226 Murnon or MANUr Ac'ic SEMI-CONDUC- TIVE DEVICES HAVING ELECTRODES CON- TAEJING ALUMINUM Nico Bram Speyer, Mollenhutseweg, Nijmegen, Netherlands, assignor to North American Philips Company,

lno, New York, N.Y., a corporation of Delaware Filed June 26, M59, Ser. No. 823,037 tCiaims priority, application Netherlands, Aug. 1, 1958, 30,165 4 Claims. ((129-2531) This invention relates to methods of manufacturing semi-conductive devices, such as crystal diodes and transistors, which comprise semi-conductive bodies of germanium having at least one electrode containing aluminum.

French patent specification No. 1,128,423 already discloses that the electrical properties of semi-conductive devices may be considerably improved by adding elements, such as gallium and aluminum, to the conventional metals from which electrodes are manufactured, such as indium. Such an improvement is obtained more particularly for transistors if aluminum is added to the material of the emitter.

It is also known that the addition of aluminum causes difficulty since aluminum and alloys containing aluminum, even if the aluminum content in the latter is very low, are spontaneously covered in air with an oxide film by which fusing together wih another element or alloy is impeded.

Several methods have previously been suggested for overcoming this difiiculty (see, for example, the article by Armstrong, Carlson and Bentivegna in R.C.A. Review Vol. 17 (1956), number 1, especially page 39). According to a first solution, at first an electrode is applied by fusion to the germanium, which'electrode consists of a metal or an alloy which readily fuses together with germanium, such as indium. Subsequently, a small amount of an alloy of indium and aluminum is provided on the electrode, whereafter the two metals are fused together. Two further solutions are given, which are irrelevant in this specification.

It has been found that the above-described method not always yields satisfactory results, since indium and alulil'llllll do not mix up at the conventional fusing temperature of about 500 C.

The present invention also relates to such a method of providing electrodes containing aluminum, in which at first an amount of metal free of aluminum is applied by fusion and, subsequently, aluminum is added thereto. In this connection, it is to be noted that the indication metal is to be understood in this specification to mean also an alloy.

An object of the invention is inter alia to provide a simple method which permits of obtaining electrodes containing aluminum and also uniform results.

According to the invention, a part either consisting of aluminum or containing aluminum is immersed into the fused metal free of aluminum while this metal is still in the molten state, whereafter the whole is subjected to a thermal treatment of a duration such that the aluminum dissolves at least in part.

The invention underlies recognition of the fact that 3,18l ,ZZd Patented May 4, 1965 such a part of aluminum, after having been immersed, upon further heating is protected against further oxidation. It is also based upon the recognition that the natural oxide film spontaneously formed on aluminum at room temperature usually does not impede dissolution of the aluminum in the metal applied by fusion, if both metals can react upon each other for a sufficiently long time.

In this respect, the present application is distinguished from the known method in which a small amount of an alloy containing aluminum is provided on the metal already applied by fusion. In this method, the contact between the two amounts of metalis usually so loose that a strong oxide film can readily be formed during heating so that in many cases fusing together does not take place.

A favourable circumstance in the method under consideration is the presence of germanium in the metal applied by fusion, which germanium enhances dissolution of the aluminum. The germanium may either be added to the metal prior to its application by fusion, or it may be absorbed from the germanium body during the fusing process.

The duration of the thermal treatment, apart from the minimum previously mentioned, is not critical and may be determined empirically. It is a function of the kind of the natural oxide film present on the aluminum and upon temperature. In the technique under consideration, it is common practice to manufacture large numbers of semi-conductive devices simultaneously, the duration of heating being chosen so long that aluminum has dissolved in substantially all of them. In most cases, a treatment of 10 minutes suffices.

The part to be immersed may either be the end of an aluminum wire, or a wire covered with aluminum at least at its end.

A part of aluminum which is immersed into molten metal usually has a tendency to float on the surface of the metal as a result of the action of the surface tension of this metal which does not moisten the natural oxide film of the aluminum. The wire may then be chosen of a length such that it is sui'liciently heavy to eliminate the action of the surface tension.

It has been found desirable to choose the thickness of such a wire between 200 and 300 microns. If the wire is chosen unduly thin, it is difiicult to handle, but the process of dissolving the aluminum is still satisfactory. If the wire is chosen unduly thick, the risk may be involved that a comparatively large amount of germanium dissolves in the added amount of aluminum, resulting in the geometry of the semi-conductive device being disturbed, more particularly in the vicinity of the end of the aluminum wire introduced into the metal applied by fusion.

The term Wire of a given thickness is to be understood to mean not only a circular wire having this specific diameter, but also a wire of st-ripform having a width greater than its thickness.

If the aluminum is added in the form of a homogeneous wire, a piece thereof projects from the electrode after the thermal treatment. Since the alloy formed by the aluminum with the germanium dissolved in the metal applied by fusion is very brittle, said end of the Wire may readily be removed.

As a result of this circumstance, it can readily be tested whether the aluminum has actually been absorbed into the electrode. In those cases in which this should not occur, the Wire retains its rigidity and cannot be broken off.

If the aluminum is added in the form of a wire comprising a core of a different metal and a sheath of aluminum, the core is preferably chosen so that, after the thermal treatment, it retains its rigidity and is wetted by the metal applied by fusion, so that it serves not only for immersion of the aluminum, but also as a current supply wire. The core may consist, for example, of nickel. Molybdenum and tungsten are also serviceable.

If desired, two or more wires, preferably one homogeneous aluminum wire and a current supply wire, may jointly be introduced into the metal applied by fusion.

In order that the invention may be readily carried into effect, several embodiments will now be described more fully with reference to examples which are clarified by means of the accompanying diagrammatic drawing, in which:

FIGS. 1-3, 6, 8, and 9 show diagrammatically, in section, several stages of a method according to the invention;

FIGS. 4, 5 and 7 are side views of semi-conductive devices;

FIG. 10 is a side view of a wire for use in such a method.

Example I A template (see FIG. 1) consisting of a small block of graphite 1 and provided with a bore 2 contains a germanium body 3 of the n-conductivity type, on which bears a graphite plug 4 having a bore 5 of, for example, 4 mms. in diameter.

A pellet 6 consisting of an alloy of indium and 0.5% of gallium is introduced into the bore 5. The composition of the pellet is not critical per se, but the pellet is required, and this applies to all electrode materials, to satisfactorily adhere to the germanium upon fusion. Other serviceable materials are lead, tin and bismuth. As previously mentioned, said materials may be alloyed with germanium. It is known to adopt this method if deep penetration of the electrode into the germanium is to be prevented.

The template is now placed in an oven (not shown) and heated at 500 C. in an atmosphere of hydrogen for 10 minutes. It is possible to add to the hydrogen a little hydrochlorine which acts as a flux and improves adhesion of the alloy to the germanium. The pellet then acquires the shape of the electrode 7 shown in FIG. 2.

The template is then removed from the oven and, while the electrode 7 is still in the molten state, a piece of aluminum wire 8 having a thickness of 0.2 mm. and a length of 1 cm. is immersed into it (see FIG. 3) Subsequently, the template is replaced in the oven.

In this connection, it is to be noted that in this branch of engineering it is common practice to use multiple templates in which a large number of germanium bodies are provided with electrodes simultaneously. The thermal capacity of such multiple templates is usually sufficient for the electrodes 7, after the templates have been removed from the oven, to be maintained fluid so long that all the wires can be immersed. If the templates should cool down too rapidly, they may be placed on a heated substratum.

After the templates have again been heated at 500 C. for about 10 minutes, they are removed from the oven and, after cooling, the germanium bodies are taken out of the templates. The part of the Wire 8 projecting from the electrode 7 can now readily be removed (FIG. '4). For obtaining a transistor, a conductor, for example a nickel strip 9, is secured by fusion to the electrode 7 which is intended as an emitter. The germanium body 3 is soldered to a gilt molybdenum carrier 11 with the aid of a thin layer 10 of indium, which constitutes the collector, while a base connection 12 is secured to the body 3 by means of tin.

Example II The method as described hereinafter is a variant of the preceding, in which at the same time with the immersion of the aluminum wire 8, a nickel wire 15 is immersed into the metal 7 provided by fusion (see FIG. 6).

After the aluminum has been dissolved, the wire 15 only remains in the electrode (FIG. 7). In order to enhance central positioning of the wire, it is possible to provide a guide plug 16 in the bore 5 of plug 4, which plug 16 may likewise consist of graphite (see FIG. 8).

Example III In a similar manner as described in Example I, a wire 13 consisting of a nickel core 19 and an aluminum sheath 2%) may be immersed into the electrode material 7 applied by fusion. After this aluminum, insofar it was immersed, has dissolved, the part of the Wire projecting from the electrode may be used for the current supply, as shown on an enlarged scale in FIG. 9.

Example IV Similarly, a nickel wire 22, the lower end of which has wound on it a piece of aluminum wire 23 (see FIG. 10), can be immersed into the electrode material applied by fusion.

It will be evident that several further variants are possible Within the scope of the invention.

What is claimed is:

l. A method for making an aluminum-doped contact to a germanium semiconductive body, comprising providing in contact with the body a mass of metal free of aluminum but capable of alloying with the body, heating the assembly of body and mass at a temperature of about 500 C. at which the metal mass is melted, thereafter, while the mass remains in a molten state, immersing in the melt beneath its surface a wire-like member containing aluminum, thereafter subjecting the resultant assembly to a heat treatment at which at least some of the aluminum dissolves in the molten mass, thereafter cooling the assembly to solidify the mass and form the aluminum-doped contact alloyed to the body, and thereafter removing the portions of the wire-like member projecting from the solidified mass.

2. A method for making an aluminum-doped contact to a germanium semiconductive body, comprising providing in contact with the body a mass of metal free of aluminum but capable of alloying with the body, heating the assembly of body and mass at a temperature below the melting point of germanium and at which the metal mass is melted, maintaining a wire-like member containing aluminum, having the natural oxide film spontaneously formed on aluminum at room temperature, in an unheated state, thereafter, while the mass remains in a molten state, immersing at least a portion of said unheated wire in the melt beneath its surface, thereafter subjecting the resultant assembly to a heat treatment at which at least some of the immersed aluminum dissolves in the molten mass, and thereafter cooling the assembly to solidify the mass and form the aluminum-doped contact alloyed to the body.

3. A method as set forth in claim 2 wherein the wirelike member comprises an aluminum-coated core of a metal that does not dissolve in the melt and remains rigid.

4'. A method for making an aluminum-doped contact to a germanium semiconductive body, comprising providing in contact with the body a mass of metal free of aluminum but capable of alloying with the body, heating the assembly of body and mass at a temperature at which the metal mass is melted, thereafter, while the mass remains in a molten state, immersing in the melt beneath its surface a first wire-like member containing aluminum and a second wire-like member, thereafter subjecting the resultant assembly to a heat treatment at which at least some of the aluminum dissolves in the molten mass, and thereafter cooling the assembly to solidify the mass and form the aluminum-doped contact alloyed to the body from which projects the second wire-like member.

References Cited by the Examiner UNITED STATES PATENTS Moles.

Armstrong 1481.5 Kordalewski 148-1.5 Raithel 2925.3 X Adcock 2925.3 X

6 12/59 Hall 2- 29-25.3 X 12/59 Armstrong 29-25.3 8/60 Maynard et a1. 29-25.3 X 7/61 Gotzberger 2925.3 X

OTHER REFERENCES Hansen: Constitution of Binary Alloys, 2nd Ed., N. Y. McGraW-HilL 1958, pages 101, 119, 133, 1040.

RICHARD H. EANES, ]R., Primary Examiner.

THOMAS BEALL, LEON PEAR, Examiners. 

1. A METHOD FOR MAKING AN ALUMINUM-DOPED CONTACT TO A GERMANIUM SEMICONDUCTIVE BODY, COMPRISING PROVIDING IN CONTACT WITH THE BODY A MASS OF METAL FREE OF ALUMINUM BUT CAPABLE OF ALLOYING WITH THE BODY, HEATING THE ASSEMBLY OF BODY AND MASS AT A TEMPERATURE OF ABOUT 500* C. AT WHICH THE METAL MASS IS MELTED, THEREAFTER, WHILE THE MASS REMAINS IN A MOLTEN STATE, IMMERSING IN THE MELT BENEATH ITS SURFACE A WIRE-LIKE MEMBER CONTAINING ALUMINUM, THEREAFTER SUBJECTING THE RESULTANT ASSEMBLY TO A HEAT TREATMENT AT WHICH AT LEAST SOME OF THE ALUMINUM DISSOLVES IN THE MOLTEN MASS, THEREAFTER COOLING THE ASSEMMBLY TO SOLIDIFY THE MASS AND FORM THE ALUMINUM-DOPED CONTACT ALLOYED TO THE BODY, AND THEREAFTER REMOVING THE PORTIONS OF THE WIRE-LIKE MEMBER PROJECTING FROM THE SOLIDIFIED MASS. 